Xenodiagnosis: a method for pathogen detection

As an ecologist something that I am interested in understanding more about are the factors that affect distribution and abundance of species. The environments of interest could be within a lake, within a woodlot, or even within a population of host species. However, before data can be collected about the factors influencing species distribution and abundance, the presence of those species within the environment of interest needs to be determined. Techniques have been developed that determine presence of species. For example,

mist nets are used to determine the presence and abundance of bird and bat species,

Trap set in shade of shrub. This protects the trap from direct sunlight and overheating. Photo by Adam Green (http://www.biosbcc.net/bio130/Mammal%20Trapping.htm)

and there are specific types of traps to catch lobsters. There have been methods developed to trap most animals. But what if the species you are looking for is a pathogen? What if this pathogen is not easily detected when the host is directly tested?

Pathogens typically live only within organisms, so a different approach must be used to determine if they are present. Molecular methods must be used in these cases, but it is not always as easy as using the polymerase chain reaction (PCR) method targeting a gene in the pathogen. In hosts, especially patients and some wildlife, when tissue or blood is tested using the polymerase chain reaction (PCR) method, many times the test comes up negative when the host is infected (this is referred to as a false negative result). This is true for pathogens such as the bacterium that causes Lyme disease (Borrelia burgdorferi) and the protozoan that causes Chaga’s disease (Trypanosoma cruzi). To help decrease the amount of cases in which this incorrect result occurs, a method was described in 1914 called xenodiagnosis. This method was developed to test patients for infection with trypanosomes that cause Chaga’s disease. Trypanosomes are transmitted from one vertebrate host to another through the vector the Kissing bug (Triatoma species).

A man undergoes “xenodiagnosis” for Chagas disease. German Pomar/Reuters. From http://theconversation.com/explainer-what-is-chagas-disease-40047

This method involves allowing uninfected nymphal kissing bugs to feed on a host of interest, in this case patients. Then let a certain amount of time pass in order to allow the pathogen to develop in the vector, in this case between 30 to 45 days. Finally, each bug is tested using a molecular technique such as PCR to determine if the bug is infected. As you might guess, if the bug is infected the host is infected. By using multiple vectors, the likelihood of detecting the pathogen when it is present increases substantially and the false negative rate decreases.

Since this method was developed, its uses have been expanded to address questions about other pathogens. One of particular interest to me is the causal agent of Lyme disease, Borrelia burgdorferi. In our lab, we are presently using xenodiagnosis in order to determine how many mice in several sites within a forest are infected with this bacterium. Many times when blood or tissue is collected from a mouse and tested by running a PCR, a mouse will turn out negative when it is, in reality, positive. This is why we are using xenodiagnosis to test these mice. In this case, we are testing ticks that have fed on mice to determine infection. Through knowing how many mice are infected we can begin to determine the prevalence of that pathogen in our study sites. Also, the xenodiagnosis data in conjunction with other ecological data, we are able to address questions about how infection affects the life history of the mouse itself. If mice are infected with the bacterium, do they live longer? Do they reproduce less? Do they take more risks in the environment? Having a method of detection of pathogens such as these, can help to understand more about the distribution and abundance of these species and also how they affect the host species. Over time, the factors that affect distribution and abundance can be addressed as well.